The present invention relates to a system and method for controlling the performance of an exhaust gas sensor. More specifically, the invention pertains to controlling a self-heated sensor based upon the impedance of the sensor.
State and national regulations for vehicle emissions have required engine manufactures to maintain tight control over engine performance. As a result, most modern internal combustion engines utilize an electronic control module (ECM) that receives data from various sensors throughout the engine. The ECM then generates signals controlling various components of the engine.
In order to maintain optimum engine performance while reducing noxious emissions, it is highly desirable to control the air/fuel mixture provided to the engine within stoichiometric ratios. In certain applications, it is desirable to maintain the air/fuel mixture at a specific lean-burn ratio. In most electronically controlled engines, this ratio is determined in relation to the detected intake air quantity, detected engine speed and a base fuel amount, as well as data from the engine sensors. In more sophisticated engine control systems, a corrected fuel amount is generated based upon the gas content of the engine exhaust.
More specifically, the gas content of the engine exhaust provides a measure for engine combustion performance, which in turn is a function of the air/fuel mixture being provided to the engine. For example, if the air/fuel mixture is rich, the engine exhaust will include a lower than normal concentration of oxygen (O2). On the other hand, if the air/fuel ratio is too lean, NOx emissions in the engine exhaust increase.
In order to accurately regulate the air/fuel mixture to acceptable proportions (e.g., stoichiometric or lean-burn), most engines include an exhaust gas sensor disposed within the engine exhaust conduit. Most typically, the sensor is an oxygen sensor that generates an output signal in relation to the concentration of oxygen (O2) passing through the sensor.
One problem encountered by a typical exhaust gas oxygen sensor is that the output of the sensor is temperature dependent. For most internal combustion engines, the exhaust temperature can vary significantly, which can lead to detrimental perturbations in the sensor output that may not be indicative of changes in O2 concentration. The effect of temperature variations can be more significant in lean-burn applications.
In order to address this problem, most exhaust gas sensors are self-heated, meaning that a heating element is disposed immediately adjacent the sensor to elevate its temperature. When the sensor is calibrated at the elevated temperature, the normal temperature variations of the exhaust gas flow have a much less significant impact on the accuracy of the sensor output. For instance, exhaust gas temperatures will typically range from ambient to about 400xc2x0 C. In many known exhaust gas oxygen sensors, the heating element heats the sensing element to 800-900xc2x0 C.
Even with this improvement, the accuracy of the output of most known exhaust oxygen sensors is less than optimum. There remains a need in the industry for an exhaust gas sensor that can maintain accurate gas level readings in spite of variations in the engine exhaust temperature.
In order to address this need, the present invention contemplates a system and method for controlling or adjusting the accuracy of an exhaust gas sensor utilizing the impedance of the sensor. In one embodiment, a periodic AC signal is superimposed on the low frequency or DC voltage signal output of the gas sensor. The AC current flowing through the gas sensor is a function of the actual impedance of the sensor, which is in turn a function of the temperature of the sensor. Thus, the invention further contemplates an impedance sensor circuit connected to an output of the gas sensor. The output of the impedance sensor circuit is a peak voltage that is indicative of the AC voltage drop across the sensor, and ultimately the impedance of the sensing element.
In a further aspect of the invention, this peak voltage is utilized to control the operation of the heating element. In particular, the invention contemplates a closed loop control system in which the thermal output of the heating element is continuously varied to accurately maintain a consistent temperature for the exhaust gas sensor. In one preferred embodiment, a heater controller provides a variable voltage signal to the heating element. The thermal output of the heating element then varies in proportion to the magnitude of the voltage provided by the heater controller. In a further feature of the invention, the heater controller can receive control signals from an engine control module to which the impedance signal of peak voltage value is provided. The engine control module can include software or circuitry that translates the impedance peak voltage value to a voltage to be applied by the heater controller to the heating element. Thus, using this active closed loop control, the thermal output of the heating element may vary widely, while the actual impedance of the exhaust gas sensor should remain fairly constant.
In a specific embodiment of the invention, the impedance sensor circuitry includes a bandpass filter centered around the frequency of the AC signal superimposed on the voltage signal generated by the gas sensor. More particularly, the circuitry includes a high-pass filter, a low-pass filter, and an amplifier. The output from the bandpass filter is provided to a half-wave rectifier, the output of which is the peak voltage signal indicative of the sensor impedance.
It is one object of the invention to provide an exhaust gas sensor that generates an accurate measure of a gas concentration. A more specific object is accomplished by features that allows the gas sensor to generate an accurate measurement value in spite of fluctuations in the exhaust gas temperature.
One benefit of the present invention is that the temperature seen by the exhaust gas sensor can be accurately controlled and maintained using gas sensor impedance feedback in a closed loop control system. Another benefit is that a minimum number of additional components are used.
A more general benefit of the present invention is that interactive control of the accuracy of the exhaust gas sensor means tighter control of the engine air/fuel ratio, which ultimately allows the engine to meet stricter emission requirements and enjoy greater reliability. These and other objects and benefits of the invention will become apparent upon consideration of the following written description and accompanying figures.